The present invention relates to an objective lens which is installed in a device employing multiple types of light beams having different wavelengths, such as an optical information recording/reproducing device for recording data to and/or reproducing data from multiple types of optical discs differing in data density.
There exist various standards of optical discs (CD, DVD, etc.) differing in data density, protective layer thickness, etc. Meanwhile, new-standard optical discs (HD DVD (High-Definition DVD), BD (Blu-ray Disc), etc.), having still higher data density than DVD, are being brought into practical use in recent years to realize still higher information storage capacity. The protective layer thickness of such a new-standard optical disc is substantially equal to or less than that of DVD. In consideration of user convenience with such optical discs according to multiple standards, the optical information recording/reproducing devices (more specifically, objective lenses installed in the devices) of recent years are required to have compatibility with the above three types of optical discs. Incidentally, in this specification, the “optical information recording/reproducing devices” include devices for both information reproducing and information recording, devices exclusively for information reproducing, and devices exclusively for information recording. The above “compatibility” means that the optical information recording/reproducing device ensures the information reproducing and/or information recording with no need of component replacement even when the optical disc being used is switched.
In order to provide an optical information recording/reproducing device with the compatibility with optical discs of multiple standards, the device has to be configured to be capable of forming a beam spot suitable for the particular data density of the new disc (in the switching of the optical disc to the new disc of a different standard) by changing a NA (Numerical Aperture) of the light beam employed for the information reproducing/registering, while also correcting spherical aberration which varies depending on the protective layer thickness. Since the diameter of the beam spot can generally be made smaller as the wavelength of the beam gets shorter, multiple laser beams having different wavelengths are selectively used by the optical information recording/reproducing device depending on the data density of the disc. For example, for DVDs, a laser beam with a wavelength of approximately 660 nm (shorter than approximately 790 nm for CDs) is used. For the aforementioned new-standard optical discs, a laser beam with a wavelength still shorter than that for DVDs (e.g. so-called “blue laser” around 408 nm) is used in order to deal with the extra-high data density.
As a technique for converging a light beam onto the record surface of each of the multiple types of optical discs, a technique of configuring an objective lens to have, on one of surfaces thereof, an annular zone structure including ring-shaped minute steps has been proposed. According to this technique, the light beams having different wavelengths can suitably be converged onto respective the record surfaces of the multiple optical discs.
Japanese Patent Provisional Publication No. 2004-247025 (hereafter, referred to as JP2004-247025A) discloses an objective lens configured to have the compatibility with the three types of optical discs such as CD, DVD and HD DVD.
In an optical pick-up disclosed in JP2004-247025A, three types of collimated light beams having different wavelengths are used so that the objective lens has the compatibility with the three types of optical discs having different densities and has the function of correcting the spherical aberration.
However, the optical pick-up disclosed in JP2004-247025A has a drawback that undesired diffraction order light having substantially the same light amount as that of the normal diffraction order light used for information recording and information reproducing is generated. For this reason, there is a demand for an objective lens enabling the optical information recording/reproducing device to be capable of accurately executing the information recording and the information reproducing while maintaning the function of accurately recording information to and/or reproducing information from the multiple types of optical discs.